Wide band frequency modulation receiving system



w. R. KocH 222,973

WIDE BAND FREQUENCY MODULATIN RECEIVING SYSTEM may a2, "1942.,

Filed June? 29, 1940 '3m emo: l #ming Patented May 12, 1942 usaranvsrarss PATENT vorgies `WIDE BAND FREQUENCY MODULATION RECEIVING SYSTEMWinfield R. Koch, Haddonfield, N. J., assgnor to Radio CorporationofAmerica, a corporation of Delaware Application June 29, 1940, SerialNo. 343,137

GCI. Z50-20) Claims.

This invention relates to wide band frequency modulation receivingsystems, and has for .its

primary object to provide an improved receiving system, providing noiseand interference reduction with wide band frequency modulation.

It is a further object of the present invention 4to provide a frequencymodulation receiving-system having automatic band width control withwide band operation, and means therein for preventing audio frequencydistortion.

It is a further object of the present invention to provide a frequencymodulation receiving system having awide band signalinput circuit andtion to provide an improved wide band frequency modulation signalreceiving system having means thereinrfor varying the effective bandwidth of the intermediate frequency circuits, with variations inpercentage modulation or frequency swing, and in accordance withvariations in the audio frequency outputof the system'as a measure ofthe .frequency swing.

It isalso an object of the present invention to provide means in afrequency modulation signal receiving system for varying a heterodyneoscillator frequency to maintain a received signal within theintermediate frequency pass band with variations in modulation above apredetermined percentage or frequency swing, While permitting theVfrequency swing or modulation to occupy at all times a major portion ofthe intermediate frequencypass band width.

For lowest background noise and minimum interference from othersignalsin a frequency modulation receiving system, it is desirable to :providea signal band width substantially equal to the pass band of the system.In accordance with the invention, for this purpose.' instead of varyingthe intermediate frequency band width, the effective band width of thesignal is altered -by feedback voltage control through the heterodyneoscillator to cause the intermediate frequency signal to be maintainedat all times at a band width covering a major portion of the pass bandwidth of the intermediate frequency .circuitsfor example, eighty percentof the band width, and to cause the intermediate frequency swing orsignal band width to be maintained Within the band `width limits oftheVintermediate ,frequency circuitsfor higher percentage modulation,without causing distortion of the audio `frequency output signal.

It is, therefore, anobject of the invention to provide `means 'wherebythe audiov frequency amplification may be changed .automatically withvariations in the modulation or signal frequency swing above .apredetermined value, preferably in the intermediate .frequency circuits,to secure the proper audio frequency output level and dy- -namicoperating range -of the signal at the output circuit of the receivingsystem. IThis contenu` plates that both the effective band Width ofreception and the audio frequency gain in the system may be'controlledautomatically in response to variations in anaudio vfrequency controlvolt- `age vas `a measureof the percentage modulation or frequencyswing, thereby Ytopermit ythe effective band width to vary without audiofrequency distortion.

It is a still further lobject ofthe present invention to provide animproved frequency modulation receiving system having a Wide band sig-Vnal input circuit and a relatively narrow band intermediate frequencycircuit Yand control means .for utilizing a major portion of the bandwidth fof said last-named circuit for lo-w modulation signals thereby toreduce noise and, with feedback control of the oscillator frequency insync'hronism with the incoming signal, to prevent the intermediatefrequency swing from exceeding the band Width o-f the intermediatefrequency circuit with full modulation.

While negative feedbackfcontrol of the oscillator frequency may beprovided to narrow the signal band, it may be desirable with lowmodulation levels to utilize positive feedback to widenv the band of thesignal to equal the pass band of the receiving system, in which'fcasethe. audio frequency gain of the system should be correspondinglyreduced to prevent distortion. 4

' The invention will better 4be understood, `however, from the followingdescription, vwhen considered in connection vwith the accompanyingdrawing, and its scope is pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of a signalreceiving system-embodying the invention, and

Figure 2 is a graph showing curves illustrating certain operatingcharacteristics of the circuit of Fig. 1.

Referring to Fig. 1, the wide band frequency modulation receiving systemshown comprises a wide band tunable r. f. or high frequency amplifier 5coupled to a suitable signal source such y In the receiving systemdescribed, the amplifier stages I3 is a gain control amplifier stagewhich is preferably of the pentagrid type, having a signal input grid I6coupled through a potentiometer I'I, and an impedance coupling networkI8, to the output circuit I9 of the preceding amplier stage I2. A gaincontrol grid is also provided in the tube I3 as part of a volume controlor expander means to control amplitude of signals transmitted throughthe stage. Variable gain controlling potential for the grid 2U isderived througha control circuit 2| from the output impedance 22` of arectifier 23, a suitable filter network 24 being included in theconnection between the circuit 2| and the output impedance.

The rectifier 23 and the associated circuits include a suitable sourceof fixed biasing potential 25 for the control grid 20 and a delay biaspotential source 26 for the rectifier 23 effective to prevent responseof the rectifier to signals below a certain value of applied signalpotential corresponding to a certain percentage modulation or swing. Thepercentage modulation or frequency swing is determined by the audiofrequency o'utput of' the receiving system a portion of which is appliedto the rectifier 23 from the first stage audio frequency amplifieroutput circuit I9 through a supply lead21, a coupling capacitor 28 andan input potentiometer device 29.

Audio frequency signals are also derived from the output circuit I9through the potentiometer device I'I and a variable tap connection 3Dthereon for a feedback control or variable gain amplifier stage 3|, alsoof the pentagrid type having a control grid 32 connected with the tap 30as shown, and having a gain control grid 33 connected with the biassupply lead 2| from the rectifier 23.

The output circuit 34 of the amplifier stage 3| is coupled through asuitable coupling capacitor 35 and a circuit 36 with frequency controlmeans 31 connected with the oscillator 8 in proper phase relation tocause the oscillator frequency to Vary in synchronism with the incomingsignal, above and below a normal frequency by an amount dependent uponthe adjustment of the applied signal from the tap 30 and the biascontrol potential on the gain control grid 33 of the control amplifier3|. v

The audio frequency output from the discrim- Vinator network I Il.through the second detector I I and amplifier I 2 to the tap 39provides a measure of the percentage modulation or frequency swing of areceived signal. This causes the feedback control amplier 3| to have anegative feedback action on the oscillator frequency lwhich then swingswith the signal and in synchronism therewith, thereby to cause theeffective band width of reception through the narrow band intermediatefrequency amplifier 9 to vary with the percentage modulation.

Thus, if the signal at a mean intermediate frequency of 5 mc. is onehundred percent modulated to swing i kc., the oscillator may at the sametime be caused to swing in frequency 1-75 kc., making a differentialswing of 25 kc. in the i. f. amplifier signal for one hundred percentmodulation. This tends to result in a reduction in theaudio frequencyoutput which is compensated as will hereinafter be pointed out.

With this arrangement, the intermediate frequency amplifier may providea relatively high gain and a relatively narrow frequency response, whilethe input circuit or r. f. amplifier may have a desired wide bandcharacteristic. Then, assuming further a 40 mc. signal-modulated i100kc. at one hundred percent modulation, and i2() kc. for a relatively lowmodulation of twenty percent, the intermediate frequency amplifier maybe made substantially only 50 kc. wide.

In accordance with the invention, the effective band width of the signalis altered by feedback control through the heterodyne oscillator toprovide an i. f. signal which occupies substantially eighty percent ofthe i. f. band. width with twenty percent modulation, for example, asindicated in connection with the output response curve 40 of thediscriminator network and detector,`while the remaining veighty percentchange in percentage modulation causes a relatively slight increase inthe effective use of the full band width of the amplifier 9.

In other words, signals of relatively low percentage modulation arecaused to occupy a major portion of the band width of the signalchannel, preferably in a narrow band high gain i. f. circuit andnegative feedback voltage control to the oscillator is provided andcontrolled to a degree to preventthe signal swing from exceeding thelimits of the pass band of the i. f. amplifier up to substantially fullmodulation.

In Fig. 2, the curve 4| indicates the amplitude of the input andoutputsignal of the gain control ampliiier stage I3 with relatively lowfrequency swing, such as, below twenty percent modulation, while thecurve 42 indicates the vinput voltage with one hundred percentmodulation and the resulting output signal voltage is indicated by thecurve 43. This additional outputis provided by the control rectifier 23which causes an increasing positive bias on both grids 33 and 20 vandvolume range expansion above 20 percent modulation in the presentexample.

This action prevents distortion of the audio frequency output whichwould otherwise result from altering the effective modulation of theintermediate frequency signal by variation of the oscillator frequencyand includes controlling the effective band width of reception throughthe i. f. amplier, with variations in swing of the signal and the gainof the audio frequency amplifier and feedback control pathsimultaneously, so that the output of the receiver faithfully followsthe modulation changes above the predetermined low modulation levelwithout exceeding the band width of' the amplifier. At one hundredpercent modulation, substantially the entire band is used and the audiofrequency gainis maximum.

The rectifier 23. and its associated circuits have `a time constantwhich provides for a rapid response and slow recovery characteristicthereby insuring that the system will respond to but will Vnot followthe modulation frequency changes.

From the foregoing descriptiomit will be seen that both the amount offeedback and the audio frequency gain may automatically be controlled ifrom a rectifier operated by the audio voltages derived from thefrequency modulation receiving system following the discriminatornetwork, as a measure of the percentage modulation, therebysimultaneously making the effective band width of reception vary withthe percentage modulation above a predetermined low percentage andcompensating for'the effective reduction in the audio frequency signalamplitude from the detector by increasing the gain in both .the audiofrequency and feedback control channels.

I claim as my invention:

1. In a radio signal receiving system, the combination with a signalamplifier having a predetermined fixed passband, of means for varyingthe effective band width of signals applied to said amplifier, a controlcircuit for said last-named means including an amplifier, an audiofrequency amplifier, and means for simultaneously varying the gain ofsaid audio frequency and control circuit amplifiers in response tovariation in the signal amplitude above a predetermined value.

2. In a frequency modulation signal receiving system, the combinationwith an intermediate frequency amplifier having a predeterminedfrequency passband, of means for varying the effective band width ofsignals applied to said amplifier, a control circuit for said last-namedmeans including an amplifier, an audio frequency amplifier, and meansfor simultaneously varying the gain of said amplifiers in the same sensewith variations in frequency swing of a received signal, said last-namedmeans including an audio frequency rectifier for deriving a gain controlpotential for said amplifiers.

3. In a frequency modulation signal receiving system, the combinationwith an intermediate frequency amplifier having a predeterminedfrequency passband, of means for varying the effective band width ofsignals applied to said amplier, a control circuit for said last-namedmeans including an amplifier, an audio frequency amplifier, and meansfor simultaneously varying the gain of said amplifiers in response tovariation in the signal amplitude above a predetermined value.

4. In a frequency modulation signal receiving system, the combination ofa variable frequency heterodyne oscillator, feedback control meansresponsive to the audio frequency output ofsaid system for varying thefrequency of said oscillator in synchronism with frequency variations ina received signal, said feedback means including an amplifier forfeedback voltage, and means responsive to the audio frequency output of`said system for varying the audio frequency gain in said system and theamplifier gain in said feedback control means in predetermined relation.

5. In a frequency modulation signal receiving system, the combination ofa relatively narrow band intermediate frequency amplier having apredetermined passband, a variable frequency heterodyne oscillator,feedback control means responsive to the audio frequency output of saidsystem for varying the frequency of said oscillator in synchronism withfrequency variations in a received signal, to limit the signal frequencyband to the passband of said amplifier, said feedback means including anamplifier for feedback voltage-and means responsive tothe audiofrequency output `of saidsystem forvarying the audio lfrequency gain -insaid system and the of the audio frequency output of said system isenhanced.

6. In a frequency modulation receiving sys tem, the combination of arelatively narrow band intermediate frequency amplifier circuit, meansfor causing the effective band width of reception through said amplifiercircuit to vary with percentage modulation variation above apredetermined value, an audio frequency amplifier circuit, and means forcausing the signal lgain through said last-named circuit to varyconjointly with said band width variation, and in a sense to providecompensation in the signal output of said system for the effects of saidbandV Width variation. i

7. In a radio signal receiving system, the combination of a firstdetector, a heterodyne oscillator coupled to said detector, anintermediate frequency amplifier coupled to said detector, an audiofrequency signal channel coupled to said detector, means providing afeedback audio frequency control circuit between said audio frequencysignal channel and said oscillator for varying the frequency of saidoscillator in response to variations in the amplitude of audio frequencysignals, and means for varying the gain through said audio frequencysignal channel and the gain in said feedback circuit jointly and in thesame sense in response to variations in the amplitude of audio frequencysignals above a predetermined amplitude.

8. In a frequency modulation signal receiving system, the combination ofmeans for deriving audio vfrequency output signals therefrom as ameasure of the frequency swing of a received signal, means providing avariable-gain feedback oscillator frequency-control amplifier coupled tosaid last-named means, a variable-gain 'audio frequency amplifierfor'said system, means forY adjusting the amplitude of the audiofrequency signals applied to said audio frequency amplifier and to saidvariable-gain amplifier, and means including an audio frequencyrectifier responsive to the audio frequency output of said systemA saidvariable gain amplifier to cause the oscillator Y frequency to vary insynchronism with an incoming signal, a variable gain audio frequencyamplifier for said system, means for adjusting the amplitude of theaudio frequency signals applied to said audio frequency amplifier and tosaid variable gain amplifier, and means including an audio frequencyrectifier responsive to the audio frequency output of said system forapplying to said variable gain audio frequency amplifier andto saidvariable gain control amplifier a variable gain-controlling potential.

10. In a frequency modulation signal receiving system, the combinationof an intermediate frequency amplifier providing a predetermined signalpassband, a heterodyne oscillator, frequency nel connected with saidintermediate frequency amplifier and including an audio frequencyamplifier circuit, means for deriving an audio frequency signal fromsaid circuit, means providing a path including an amplifier for feedingback said derived audio frequency signal to said oscillator frequencycontrol means to cause the oscillator frequency to vary from a normalvalue in synchronism with the frequency variation of a received signal,thereby to vary the effective l0 mined value.

7 control means for said'oscillator, a signal chanba'nd Width of thevsignal through said intermediate frequency amplier, and means forvarying the gain in said feedback path and in said audio frequencyamplifier circuit jointly with variation in the amplitude of audiofrequency signals therein as a measure of said frequency variation of areceived signal, and means for preventing operation of said last-namedmeans for signal frequency variations below a predeter- WINFIELD R.KOCH.

